This invention relates in general to vehicle brake systems, especially brake systems for heavy duty trucks, and in particular to an improved method for casting a rotatable brake component from gray iron.
Virtually all wheeled vehicles are provided with a brake system for selectively inhibiting the rotation of the wheels and, therefore, slowing the movement of the vehicle. To accomplish this, a typical vehicle brake system includes a friction brake assembly which is provided at one or more of the vehicle wheels. Upon actuation bv a driver of the vehicle through manual movement of a brake pedal and an associated pneumatic or hydraulic actuating system, the friction brake assemblies are effective to inhibit the rotation of the vehicle wheel.
Such vehicle friction brake assemblies are generally classified into two types, namely, drum brake assemblies and disc brake assemblies. In a drum brake assembly. a hollow cylindrical drum is secured to the wheel of the vehicle for rotation therewith, while a brake shoe assembly is secured to the nonrotatable component of the vehicle. The brake shoe assembly includes a pair of arcuate friction shoes which are operatively connected to a pneumatically or hydraulically actuated piston. The friction shoes are disposed within the hollow drum adjacent to the inner cylindrical surface thereof. The friction shoes are normally spaced apart from the inner cylindrical surface of the drum. When the driver of the vehicle manually moves the brake pedal, the piston is actuated to move the friction shoes apart from one another into frictional engagement with the inner cylindrical surface of the drum. As a result, rotation of the drum and its associated wheel are inhibited.
In a disc brake assembly, a rotor or disc is secured to the wheel of the vehicle for rotation therewith. while a brake caliper assembly is secured to a non-rotatable component of the vehicle, such as the vehicle frame. The brake caliper assembly includes a pair of friction pads which are operatively connected to a hydraulically or pneumatically actuated piston. The friction pads are disposed on opposite sides of the rotor and are normally spaced apart therefrom. When the driver of the vehicle manually moves the brake pedal, the piston is actuated to move the friction pads toward one another into frictional engagement with the rotor. As a result, rotation of the rotor and its associated wheel are inhibited.
in the past, drums and rotors of the type described above have been formed from gray iron using a conventional "as-cast" method. The "as-cast" method simply involved casting molten gray iron into the desired shape of the drum or rotor and subsequently cooling, followed only by cleaning and machining when necessary. Thus, the "as-cast" method has been found to be desirable because it is a relatively simple and inexpensive method to perform. Also, gray iron has been found to be an acceptable matenal to use in the "as-cast" method because it provides the resultant drums and rotors with sufficient mechanical and physical properties for use in the fi-iction brake assemblies, such as hardness, strength, wear resistance, thermal conductivity, and the like.
Also, the friction shoes and pads have been manufactured from asbestos. As the use of asbestos has declined in recent years, the friction shoes and pads are now being manufactured from other materials. The materials used in these newer friction shoes and pads have been found to be more aggressive than those formed from asbestos. Consequently, the drums and rotors which have been formed from "as-cast" gray iron have been found to wear more rapidly. This is particularly a problem when the drums and rotors are used in the brake systems of heavy duty trucks, inasmuch as the load applied to the drums and rotors of such brake systems is very high. Thus, it would be desirable to provide an improved method for manufacturing drums and rotors for friction brake assemblies which retain the benefits of the "as-cast" gray iron method, yet which minimizes premature wear of the drums and rotors.